The invention relates to active liquid crystal displays, and more particularly to liquid crystal displays with improved gray-scale.
In a conventional TN (twisted nematic) mode liquid crystal display, gray-scale inversion in half tones of gray is an issue for viewing angle characteristics. In TN mode liquid crystal displays, gray-scale inversions do not easily occur in the upward and horizontal directions, but they easily occur in the downward direction. FIG. 1 shows viewing-angle dependence of relative luminance difference in the normally white mode for a conventional TN mode liquid crystal display in the downward direction. Curves 12, 23, 34 and 45 show the viewing-angle dependence of relative luminance difference between gray-scale gradations 1 and 2, 2 and 3, 3 and 4 and 4 and 5, respectively. As is apparent from FIG. 1, when the conventional TN mode liquid crystal display is observed at about 20°, the relative luminance difference of curve 34 is 0 and it is understood that the gray-scale inversion GS occurs such that the display quality markedly deteriorates from the downward direction.
U.S. Pat. No. 6,342,939 to Hirata et al., the entirety of which is hereby incorporated by reference, discloses a liquid crystal display with improved viewing characteristics, comprising a pair of opposing transparent substrates with a liquid crystal layer interposed therebetween. A transparent electrode is formed on the liquid crystal layer side of each of the substrates. The opposing transparent electrodes disposed in pairs have two or more different cell gaps at least within one display area of a pixel.
U.S. patent application Publication Ser. No. 2002/0105614 to Nakayama et al., the entirety of which is hereby incorporated by reference, discloses an array substrate of a liquid crystal display. In the array substrate, a second pixel electrode is disposed in a layer above an insulating layer which is itself disposed in a layer above a first pixel electrode. The second pixel electrode is electrically connected to the first pixel electrode and has a region that does not overlap the first pixel electrode, improving viewing angle characteristics.